In a conventional communication system, a data transmission delay time between devices that perform communications is measured. In a radio communication system that conforms to a common public radio interface (CPRI) standard, measurement of a T14 delay between a radio equipment control (REC) device and a radio equipment (RE) device has been known (see Japanese Laid-open Patent Publication No. 2008-11498).
The T14 delay is referred to as a time from when a frame which is the basis is transmitted from the REC device to when the frame is relayed to a terminal RE device and then received by the REC device again. The T14 delay is measured by each of the REC devices within the above described system. An adjustment of making the T14 delays of the REC devices uniform is performed by forcibly extending the T14 delay of each REC device to be equivalent to the longest one among the T14 delays measured by the REC devices, that is, by adding time to a shorter delay time. Such an adjustment is desirable for preventing any failure at the time of handover of the mobile terminal.
A technique of measuring the T14 delay is defined in the CPRI standard. A T14 delay measuring technique stated in the CPRI standard (v3.0) will be explained below with reference to FIGS. 10 and 11.
A case in which a REC device 1100, a RE device 1200, and a RE device 1300 are connected in series as illustrated in FIG. 10 is explained as an example. In this case, the T14 delay is referred to as a time from when a frame which is the basis is output from an output terminal R1 of the REC device 1100 to when the frame is input to an input terminal R4 of the REC device 1100 via an input terminal RB2 and an output terminal RB1 of the RE device 1200, an input terminal R2 and an output terminal R3 of the RE device 1300, and an input terminal RB4 and an output terminal RB3 of the RE device 1200. That is, the T14 delay is “T12(1)+T delay DL(1)+T12(2)+T offset(2)+T34(2)+T delay UL(1)+T34(1).”
T12(1) is referred to as a transmission time from when the frame is output from R1 to when the frame is input to RB2. T delay DL(1) is referred to as a processing time until the RE device 1200 outputs the frame input from RB2 through RB1. T12(2) is referred to as a transmission time from when the frame is output from RB1 to when the frame is input to R2. T offset(2) is referred to as a processing time until the RE device 1300 outputs the frame input from R2 through R3. T34(2) is transmission time from when the frame is output from R3 to when the frame is input to RB4. T delay UL(1) is referred to as a processing time until the RE device 1200 outputs the frame input from RB4 through RB3. T34(1) is referred to as a transmission time from when the frame is output from RB3 to when the frame is input to R4.
The frame input to RB2 of the RE device 1200 is branched at RB2 and is output from RB1 and output from RB3. T offset(1) is referred to as a processing time until the RE device 1200 outputs the frame input from RB2 through RB3. Therefore, in the REC device 1100, even though a time from when the frame which is the basis is output from R1 to when the frame is input to R4 is simply measured, the measuring result is not the T14 delay but a time from when the frame which is the basis is output from R1 of the REC device 1100 to when the frame is input to R4 of the REC device 1100 via RB2 and RB3 of the RE device 1200.
The RE device 1200 performs marking by giving information at a timing when the frame that is branched at RB2 and output from RB3, and the frame serving as the basis, that is branched at RB2, output from RB1, input to RB4 via R2 and R3 of the RE device 1300 and then returned, are superimposed on each other.
That is, as illustrated in FIG. 11, a frame 1400 as the basis output from R1 is input to RB2 after T12(1). The frame 1400 is output from RB1 after T delay DL(1). At this time, one frame 1400 branched at RB2 is output from RB3 after T offset(1). Returning to the frame 1400 output from RB1, the frame 1400 is input to R2 after T12(2). The frame 1400 is output from R3 after T offset(2). The frame 1400 is input to RB4 after T34(2). The frame 1400 is superimposed on one frame 1400 branched at RB2 at a timing indicated in FIG. 11 after T delay UL(1) and output from RB3. The frame 1400 is input to R4 after T34(1). As described above, in the T14 delay measuring technique stated in the CPRI standard (v3.0), the REC device 1100 measures the time from when the frame 1400 is output to when information given to the frame 1400 that is input to RB4 and returned is detected as the T14 delay.